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July–August 1914Reputable sourceWell documented

The July Crisis and the Outbreak of War

On the timeline · around July–August 1914 · The July Crisis and the Outbreak of War1915

What happened

Backed by Germany, Austria-Hungary declared war on Serbia on 28 July. Europe's alliance system pulled in the great powers within days: Germany declared war on Russia on 1 August and on France on 3 August.

Why it matters

A regional Balkan quarrel became a continental war in barely a week, showing how the rival alliance blocs turned a single crisis into a general conflict.

Sources

The July Crisis and the Outbreak of War — World War I · SourcedStory